People who think that Final Fantasy wasn't 'dark' enough before have clearly never played through key games in the franchise. The entirety of FF9 deals with the concept of free will, genocide on multiple levels, your own impending death and how you cope with knowing one day you're just going to lay down and never get up again, whether or not your life even has MEANING, the fact that the souls of the dead legitimately power the most meaningful and country advancing technology in the setting, and so many other depressing subjects that would be considered one of the darker JRPGs if not for it's cartoon art style. The art style and humor which are literally there to counterbalance the sheer existential dread permeating throughout the entire narrative.

In regards to FFXVI, like Crushnight I feel that while it looks more 'dark' upfront due to surface level aesthetics I don't doubt that it'll still have the same level of camp and charm as most of the rest of the series. It just might have to counterbalance all that so it doesn't have the tonal dissonance of a steak slapped on top of a glitter sprinkle cupcake.